Physics-Amps

[*mystical*]

If the headlamps use 3.5A each, how many hours should it take both the headlamps to totally drain a 40 amp hour battery?
thanx

x

[Hippy]

3.5 by two = 7 Amps. For a 40 Amp hour battery you would think that 40 divided by 7 = 5.7 hours would be the answer. However, as the battery starts to run down the voltage delivered reduces therefore, the Amps flowing in the now dimming bulbs will be less, hence increasing the time before the battery is completely exhausted, or the lamps fail to glow, whichever is your measure of the "totally drain" (ing) of the battery.

In any event it is likely to be much less than the 5.7 hours suggested by the arithmetic, because the 40 Amp hours refers to the amount of charge it is possible to put into the battery, not necessarily the amount you get out. Some power is lost in heat, conversion of electrolyte to gas etc.

As a guide, you can safely assume that the car starter motor consumes about 40 Amps. Now who could expect to turn the engine over on the battery for an hour? It would become exhausted after about five or six minutes of that sort of punishment. Leave the headlights (and side/tail ones as well) switched on and the battery will be need charging after around two hours.

[KebabMeister]

Also, in practice, for the headlamps to be on fully then the ignition would need to be on and therefore causing more voltage drain (ignition off with headlights on brings on a dim/dip relay which reduces the ampage going to the headlamps and therefore saving power).

Of course you have a paper based question with no other issues taken into account.